1. Field of the Invention
This invention relates to circuits which convert voltage input to duty cycle output and more particularly to such a circuit in which the output varies linearly with the input.
2. Description of the Prior Art
Voltage-to-frequency or voltage-to-duty cycle conversion is often desired because the frequency or duty cycle is less susceptible to noise than an analog voltage. In some cases, the frequency or duty cycle is received by a microprocessor and converted in software to a voltage variable for use within the microprocessor. The use within the microprocessor is greatly simplified if the frequency or duty cycle is linearly proportional to the original input voltage. However, since voltage-to-frequency conversion circuits tend to be overly complex and expensive, a voltage-to-duty cycle conversion is preferred and, accordingly, the need for a simple, linear and inexpensive voltage-to-duty cycle converter has existed.
The present invention provides a low cost and simple voltage-to-duty cycle conversion that is linear with input voltage. The present invention provides a circuit that operates on the input voltage in a predetermined manner to create an output that varies along a first ramp from a first voltage to a second voltage and then back along a second ramp from the second voltage back to the first voltage. This output is presented to a hysteresis comparator to create a periodic pulse train the duty cycle of which is linearly proportional to the input voltage. The values of certain circuit components can be changed so as to change the slope of the ramps and thus the relationship between the output duty cycle and the input voltage. In accordance with the present invention, the linear relationship will, nevertheless, be maintained.